Technical Abstract:
Satellite estimates of net primary productivity (NPP) provide continuous, long-term records of vegetation response to both natural and anthropogenic changes in global processes. Estimates are often derived from light use efficiency models that exploit biophysical relationships between the normalized difference vegetation index (NDVI) and the fraction of photosynthetically active radiation absorbed by plants (fAPAR, 0.4 to 0.7 um). We examined these relationships for cotton, wheat, and grain sorghum crops grown at Maricopa, Arizona, using the Free Air Carbon Dioxide Enrichment (FACE) facility wherein undisturbed plant canopies could be measured without the microclimatic and structural artifacts usually associated with chamber-based research. Our investigations using wide- and narrow-band radiometers revealed minimal direct effects of elevated CO2 on the NDVI versus fAPAR relationship, validating the use of this relationship for monitoring global agricultural productivity from remotely-sensed data in a rising CO2 world.